1. Field of the Invention
This invention relates to a method of producing fiber reinforced matrix composites, to methods of using such composites, and to articles prepared from such composites. It is a result of a contract with the Department of Energy Contract No. W-7405-Eng:36.
Modern carbon and graphite yarns exhibit extremely high tensile strengths and Young's modulus values on the order of about 500 GPa. Thus, a great deal of interest in the use of such fibers has resulted. One particular area of interest is in high temperature applications of such fibers after they have been coated with ceramic materials (i.e., materials made by the baking or firing of nonmetallic material). In the formation of fiber reinforced ceramic matrix composites, the ceramic phase must infiltrate into the assemblage which is made up of the yarns. In two-, three-, and even one-dimensional composites, such infiltration is difficult because the assemblages are made up of a very large number of filaments with small spacings (at most, several microns) between the filaments. However, in order to obtain optimal mechanical properties using carbon of graphite fibers, it is essential that each filament (i.e., fiber) making up a strand of yarn be uniformly coated either with the ceramic phase or with a metal which can be reacted with the carbon or graphite fiber to form the ceramic phase, but leaving an inner core of carbon or graphite after such a ceramic-forming reaction. Also needed is a method of economically producing very long lengths of coated carbon and graphite yarn. It is also essential that gaps or voids not be present at the interface of the ceramic and inner core in a coated fiber which is to be subjected to stresses at high temperatures, since such gaps will result in degradation of the coated fiber. Since it is known that such gaps do form when a metal coated carbon or graphite fiber is heated to a temperature sufficiently high to form a metal carbide, a method which eliminates such gaps is needed.
2. Prior Art
In the prior art, although U.S. Pat. No. 3,269,802, Wainer et al., discloses a process wherein hydrogen reacts with a metal halide at the surface of a carbonized material, that patent does not disclose a method for forming a weavable fiber but instead forms a carbide which is brittle. Additionally, that patent does not even address the problem of obtaining a uniform metal deposition throughout the bundle of tiny filaments which make up the larger yarn strand. Neither does Wainer address the problem of canning (i.e., coating only the outer and not the inner filaments of the yarn bundle), which phenomenon occurs when the pressure in the coating chamber is not below about 300 torr. Neither does Wainer teach or suggest hot pressing of a coated product.
U.S. Pat. No. 3,369,920, Bourdeau, discloses a process for uniformly depositing a coating onto various materials; however, that coating is a pyrolytic coating and requires a deposition temperature from 1300 to 2100 degrees centigrade, a temperature range which is far above the range employed in the present invention. At such temperatures, the fiber material to be coated would be damaged, and canning would occur. Furthermore, Bourdeau does not form a weavable product, but instead forms a brittle product. Additionally, Bourdeau does not teach or suggest hot pressing of a coated product.
U.S. Pat. No. 3,294,880, Turkat, discloses a process for preparing unifilamentary pyrographite, metal carbide, or ceramic filaments; however, the problem of uniformly coating the individual fibers in a multifilamentary yarn strand as described in the present invention is not mentioned or alluded to. The only temperatures recited in that patent are in the range of 1900.degree.-2300.degree. C.; and if a vapor deposition reaction were carried out in that range, canning would definitely occur. Furthermore, Turkat does not teach or suggest hot pressing of a coated product.
U.S. Pat. No. 3,991,248, Bauer, describes a process for forming fiber-reinforced composite articles by depositing pyrolytic materials onto the fibers. Here again, however, there is no teaching of how to avoid the problem of canning nor any recognition that it may be a problem. In that patent, there is no suggestion of the desirability of obtaining uniform coating of the tiny individual filaments making up each strand; instead, a uniform coating around the outside of the large strand is desired so that bonding between the strands will occur. Thus, for the purposes in Bauer, canning may indeed not be a problem. Also, in order to deposit refractory metals as pyrolytic materials, much higher temperatures would be required for the pyrolysis reaction than are used in the present invention; and at those higher temperatures, canning will occur. Additionally, in that patent, there is no teaching nor suggestion of the desirability of hot pressing in order to densify the product. Instead, Bauer performs densifying by a second infiltration of pyrolytic material. A fiber reinforced metal-coated product produced in Bauer would not be very useful at high temperatures (greater than 1300 degrees centigrade) because carbides would form at such temperatures by a diffusion reaction of the metal with the carbon substrate and thus gaps or voids would form, leading to a weakened product.
Thus, although there are methods of coating carbon and graphite fibers in the prior art, these methods have not even addressed, much less solved, the problem of coating very long lengths of a unidirectional yarn fiber bundle so that the individual filaments (including both the inner and outer filaments) making up the fiber bundle are all substantially uniformly coated throughout the bundle and so that the coating of the individual filaments at any one cross section is substantially the same as the coating at any other cross section. Furthermore, the prior art has not taught a process for obtaining such a uniform coating of fibers using refractory metals so that the coated filament is not brittle and can be woven. Furthermore, the prior art has not solved the problem of eliminating voids or gaps in unidirectional ceramic matrix composite fibers.